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Strolen's Citadel is a StackExchange-like site (in that things are voted up and down) where members submit plots, NPCs, quests, dungeons, societies, races, items, locations, nations, devices, and any other element that can be used in a roleplaying game.

The site is highly structured, so you can browse the entire list of plots or you can narrow it down by Plot Type (campaign, side-quest, encounter, etc.) and by Hook Type (coincidence, discovery, hired, etc.) by clicking on the categories in the side bar, to just see the kinds of plots that suit your immediate needs. It also allows you to sort the plots by Rating (among other ways) so that you can sift the highest-quality submissions to the top. You can also sort by author if you stumble upon someone whose submissions you really enjoy.

For more plot hooks you can go farther afield and look in the NPC, Location, Items, and Societies categories (and perhaps others). Obviously those will be more hit-and-miss, but some of the more detailed submissions (especially among the NPCs) will have built-in story hooks that you can adapt or use outright. Those are similarly structured so that you can filter them by different types and ratings.

There are plot hooks hiding everywhere on that site. Even a submission as seemingly simple as one about dungeon doors, like the Pegoran Doors submission, can contain enough plot ideas to build an entire campaign around.